1. Field of the Invention
This invention relates to a control system for a hybrid vehicle with an engine and a motor, and in particular, to a control system for a hybrid vehicle which appropriately charges a battery even when the vehicle is cruising and the engine is not assisted by the motor.
This application is based on Japanese Patent Application No. Hei 10-347543, the contents of which are incorporated herein by reference.
2. Description of the Related Art
Conventionally, hybrid vehicles which carry motors as power sources for driving the vehicles in addition to engines are known. Hybrid vehicles are divided into series hybrid vehicles and parallel hybrid vehicles. In series hybrid vehicles, the engine drives a generator, which outputs electric power to the motor, and the motor drives the wheels.
Because the engine and the wheels are not mechanically connected, the engine constantly runs within a rotational range which reduces fuel consumption and emissions, as compared with conventional engine vehicles.
In parallel hybrid vehicles, the motor connected to the engine assists the rotation of the drive shaft of the engine while charging a battery using a generator which is separate from the motor or is the motor itself.
Although the engine and the wheels are mechanically connected, the parallel hybrid vehicle can reduce the load to the engine. Thus, the parallel hybrid vehicle also has reduced fuel consumption and emissions as compared with conventional engine vehicles.
In the parallel hybrid vehicle, the motor for assisting the output of the engine is directly connected to the output shaft of the engine, and acts as a generator for charging the battery when the vehicle speed is reduced. Alternatively, either or both of the engine and the motor may generate the drive power, and the generator may be provided separately.
When a hybrid vehicle accelerates, the engine is assisted by the motor, and, when it decelerates, various operations are performed, such as charging the battery by deceleration regeneration, so as to maintain sufficient electric energy in the battery (hereinafter referred to as the "state of charge (remaining charge)") to meet the driver's demands. Specifically, because high deceleration regeneration is obtained after high speed cruising, the battery regains a part of the consumed energy when decelerating. After the vehicle goes up a slope such as a mountain path, the vehicle can charge its battery by deceleration regeneration when the vehicle goes down (as disclosed in, for example, Japanese Patent Application, First Publication No. Hei 7-123509).
In the conventional hybrid vehicles, in particular, in the parallel hybrid vehicle in which the driving shaft of the engine is connected with the motor, when the vehicle is cruising while the motor does not perform torque assist for the driving shaft of the engine, the motor acts as a generator, thereby increasing the remaining charge of the battery.
The remaining charge, however, is not always increased when the vehicle is running. The remaining charge mainly depends on the past running condition. The amount of electric power generation should be adjusted appropriately in consideration of the remaining charge.
When the remaining charge is decreasing depending on the running condition and on the road condition, the amount of electric power generation must be adjusted so as to stop the decrease of the remaining charge.